While a large number of steel refining methods are known and, in the most general sense, ferrous melts can be refined by subjecting them in a molten state to a reaction with oxygen in the presence of a slag, it has been found that a particularly effective refining method and apparatus utilizes a lance for directing a stream of an oxygen-containing gas, generally technical-grade oxygen, onto a melt of the molten metal which is overlain by a layer of slag.
While the refining of steel melts must take into consideration an extremely large number of factors of greater or lesser importance, it is generally accepted that it is desirable to include in any such melt a maximum of solid ferrous materials such as iron or steel scrap or high grade ores or ore concentrates having a high concentration of iron. To incorporate such materials into the melt it is generally necessary to subject at least the upper portion of the melt to sufficiently high temperatures that the materials will be reacted and/or smelted. Another crucial factor is the elimination of sulfur and phosphorus to the greatest extent possible. Thus, while other parameters are highly significant, considerable attention has been given to methods whereby iron-rich mineral matter and scrap metal can be incorporated in the melt and to methods whereby the sulfur and phosphorus are eliminated as rapidly as possible and to the greatest extent possible.
Top-blowing of the melt using a lance in the aforedescribed manner has been found to be especially effective for both purposes.
In such systems, it has been found to be desirable to maintain a frothy or foamy slag layer above the molten metal. A prerequisite of the formation of a frothy slag layer is the strong oxidation of the slag components. When the slag is less oxidized or deoxidized, it tends to be less frothy and more compact, whereas a high degree of oxidation results in a more frothy slag.
It is also known to bottom blow a melt by introducing a gas into the melt through passages or tubes formed in the bottom of the melt-receiving vessel.
By varying the position of the lance to raise or lower it relative to the level of the bath, for a given oxygen flow rate and a given configuration of the head of the lance, it is possible to control the distribution of the blowing oxygen between the slag and the metal and thus modify the degree of oxidation of the slag by, for example, preferentially oxidizing same to ensure a frothy consistency of the slag which promotes dephosphorization and desulfurization of the melt.
The preferential oxidation corresponds to a relatively high position of the lance vis-a-vis the melt surface. A relatively low position of the lance results in less oxidation of the slag and greater penetration of the oxygen stream into the molten metal, i.e. corresponds to increased decarbonization and release of heat primarily at the point of impact of the oxygen jet. This increased thermal energy can be desirable when it is necessary to promote fusion of scrap and other solids introduced into the melt.
However, all efforts to increase the temperature at the melt surface to promote the smelting of large amounts of scrap are counteracted to a certain degree by the presence of a frothy slag layer which acts as a thermal insulator.
Research at least in part carried out by at least one of us and described in the commonly owned Luxembourg patent 81 207 has shown that the insulating effect can be partly overcome by ensuring on the surface of the melt or in immediate proximity to the bath, a post-combustion of carbon monoxide released from the melt while the thickness of the slag layer and its consistency are controlled by injection of a gas which is essentially inert to the refining process at the bottom of the melt.
Thus while the combination of inert gas introduction from below and top-blowing with oxygen from a lance above the melt affords a significant advance in the art, problems have been discovered with this system. Under at least some conditions, the inert gas, generally because it tends to concentrate at the interface between the molten melt and the slag, maintains the slag layer in a permanent deoxidized condition. As a result, the slag does not have the desired frothy consistency and is less effective in the desulfurization and dephosphorization than is required.
Thus, the system of this Luxembourg patent only partly solves the problem.